Compared to parental or peer influences, the role of siblings in development has received little attention. Existing research has focused primarily on sibship-constellation variables such as ordinal position, sibship-spacing, and sex of child and siblings. In contrast, the current project focuses on the qualitative features of the sibling relationship--i.e., the children's perceptions of each other and their characteristic pattern of interaction. Six principal studies are planned. In the first, a self-report measure of sibling relationships is developed and initially validated. In the second study the child's, the sibling's, and the parent's perceptions of the sibling relationship are all examined. The third study focuses on the relations between perceptions of the sibling relationship and the pattern of interaction during free play, a cooperation task, and a teaching task. The fourth study examines the similarities and differences of the perceptions of sibling relationships at three developmental periods (late elementary school, junior high, senior high). Study five focuses on the pattern of correlations among the sibling relationship, parent-child relationships, and the parent-sibling relationships. In the final study, the potential effects of each of the familial relationships on the individual child's social behavior are considered. Thus, the series of studies will combine the power and sensitivity of self- and other-report measures with the objectivity of behavioral observations. This research on the qualitative features of the sibling relationship should provide information about what processes may be responsible for sibship-constellation effects. Additionally, the current project will be able to identify individual differences in sibling relationships even when the same sibship-constellation is present. Finally, we expect to demonstrate that the qualitative features are related to the children's social behavior in and out of the family more closely than the constellation variables are.